1. Field of the Invention
The present invention relates to a starting control circuit for a mechanical duty single phase alternating current motor having a start winding and a main winding.
2. Description of the Prior Art
Single phase mechanical duty alternating current electrical motors typically have both a start winding and a main winding. The start winding is activated or energized from the alternating current source in order to provide sufficient starting torque to start the motor. The start winding is then subsequently disconnected from the power source after the motor speed obtains a predetermined value and, thereafter, the main winding powers the motor.
There have been a number of previously known ways to disconnect the start winding from the power source after the motor attains a predetermined speed. However, the most common method for electrically disconnecting the start winding from the power source is to use a mechanical switch having a switch element subjected to centrifugal forces caused by the rotation of the electric motor. Once the motor obtains the predetermined speed, the centrifugal switch element opens the mechanical switch thereby disconnecting the start winding from the power source.
One disadvantage of these previously known mechanical switches is that the mechanical switches are prone to failure after extended use. Such failure can be caused not only by mechanical wear of the switch elements, but also by dirt, debris and other contaminants which accumulate on the switch elements.
There have, however, been a number of previously known electronic switches for electronically disconnecting the start winding from the power source after the motor attains a predetermined rotational speed. These previously known electronic switches have used a number of different methods for detecting the speed of the motor.
In one previously known method, one or more Hall effect transducers are positioned around the motor housing and a magnetic element is attached to the motor shaft. Circuitry connected to the Hall effect transducers then determines the rotational speed of the motor shaft and this circuitry, in turn, controls the activation of an electronic switch connected in series between the start winding and the power source. These previously known devices, however, have not enjoyed wide-spread usage or success for a number of reasons. Once such reason is that the Hall effect transducers and activating magnets are relatively expensive and thus increase the overall cost of the motor. Furthermore, these previously known start control circuits require a complete retrofit of the motor and thus cannot be easily used for repairing existing motors.
A still further type of electronic control circuit, such as disclosed in U.S. Pat. No. 4,496,895 to Kawate et al., utilizes an electronic circuit which compares the current phase of the start and main windings. It is well known that the current phase of the start winding leads the main winding during low speed revolution of the electric motor, but lags the phase of the main winding at higher rotational speeds of the motor. The Kawate et al. patent then uses this phase relationship between the start and main windings to activate a pulse transformer which, through other circuitry, electronically disconnects the start winding from the motor circuit when the phase of the start winding first begins to lag the phase of the main winding.
These previously known start control circuits which utilize the phase relationship of the start and main windings, however, have not enjoyed wide-spread acceptance or usage. One disadvantage of these previously known devices is that the pulse transformers are very expensive to obtain and thus economically infeasible. A still further disadvantage of these previously known start control circuits is that the start winding is disconnected from the motor circuit immediately when its phase lags the phase of the main winding.
The operation of the Kawate circuit is best understood with reference to FIG. 1 in which the motor torque is plotted on the Y axis as a function of percent of motor speed which is plotted on the X axis for a typical motor. Line 100 represents the motor torque when both the main and start windings are energized while line 102 represents the motor torque when only the main winding is energized.
From FIG. 1, it can be seen that, for ideal maximum motor torque operation and thus maximum motor acceleration, the start winding should be switched off at point 104, i.e. where the torque lines 100 and 102 cross each other. For the motor depicted in FIG. 1, this occurs at about 48% of the motor speed.
In virtually all cases for conventional motors, the phase of the start and main windings cross each other at a slower motor speed than point 104 in FIG. 1, i.e. where the torque lines 100 and 102 cross each other. For the motor depicted in FIG. 1, which is a split phase motor with a ten ohm start winding, the phase of the main and start windings cross each other at point 106 which is about 33% of the motor speed. Thus, using the Kawate circuit immediately switches off the start winding which results in an immediate decrease in motor torque to the point 108 shown in FIG. 1. This results in slower motor acceleration than if the motor start winding was disconnected at the ideal time, i.e. point 104 in FIG. 1.